Traditional signs have been based upon printed materials, paper, plastic, metal, etc., and are therefore not programmable. Accordingly, they are not easily changed. In an attempt to overcome this problem, electronically programmable and/or controllable signs were produced and have been in existence for many years. For example, liquid crystal diode (LCD) displays, cathode ray tube (CRT) displays, and other electrically-addressable displays will display an image in response to applied electric signals or fields. However, such signs typically require a large amount of electricity, since they must provide illumination in order to be visible to a viewer.
Various types of electric writeable media, some of which are commonly known as rotatable element displays or electric paper displays, also exist in the prior art. One example of a rotatable element display includes a polymer substrate and bichromal rotatable elements such as balls or cylinders that are in suspension with an enabling fluid and are one color, such as white, on one side and a different color, such as black, on the other. Examples of such rotatable element displays are described in U.S. Pat. No. 5,723,204 to Stefik and U.S. Pat. No. 5,604,027 to Sheridon, each of which is incorporated herein by reference in its entirety. Under the influence of an electric field, the elements rotate so that either the white side or the black side is exposed.
Another type of electric writeable media is known as an electronic ink display, such as the one described in U.S. Pat. No. 6,518,949 to Drzaic, which is incorporated herein by reference. An electronic ink display includes at least one capsule filled with a plurality of particles made of a material, such as titania, and a dyed suspending fluid. When a direct-current electric field of an appropriate polarity is applied across the capsule, the particles move to a viewed surface of the display and scatter light. When the applied electric field is reversed, the particles move to the rear surface of the display and the viewed surface of the display then appears dark.
Yet another type of electric writeable media, also described in U.S. Pat. No. 6,518,949 to Drzaic, includes a first set of particles and a second set of particles in a capsule. The first set of particles and the second set of particles have contrasting optical properties, such as contrasting colors, and can have, for example, differing electrophoretic properties. The capsule also contains a substantially clear fluid. The capsule has electrodes disposed adjacent to it connected to a voltage source, which may provide an alternating-current field or a direct-current field to the capsule. Upon application of an electric field across the electrodes, the first set of particles move toward one electrode, while the second set of particles move toward the second electrode.
Other examples of writeable media include liquid crystal diode displays, encapsulated electrophoretic displays, and other displays.
It has been well proven that electrostatic writing can be done on erasable electric writeable media by swiping an array of electrodes across the surface with a printer-like motion, depositing charge on the surface in an image wise fashion. The charge, once deposited, places a voltage across the medium and causes it to change its electro-optic condition by rotating, twisting or otherwise moving elements. One method of implementing this technology has been to use an islanded structure of isolated conductive material on the top surface of the electric writeable media. Islands of conductive material act as conducting surfaces for charge deposition that do not tribocharge with the writing array of electrodes and distribute the charge across each pixel in a relatively uniform manner.
There has recently been an effort to write on such islanded media with a single point stylus, much like a pen, which deposits charge on the surface causing the elements to of the electric writeable media to change. Stylus writing on electric writeable media has been performed using a uniform erase bar to uniformly charge the surface (at one voltage), followed by writing with a stylus at another voltage. Unfortunately, some applications do not allow for swiping the surface with an erase bar to uniformly charge the media. For example, using a moving erase bar is undesirable in an application implementing a device for capturing signatures digitally with a position sensor based on the pressure applied when using a stylus and visibly with an electric writeable medium.
Therefore, a need exists for creating electric writeable media that permit both stylus writing and electric erase and do not use a moving bar.
The present invention is directed to solving at least this problem.